1. Field of the Invention
The present invention relates generally to structures and techniques which are useful in high bandwidth laser communications equipment, laser radar, visible and infrared remote sensing equipment, long range, and to methods of making and using such systems.
2. Related Art
Wideband (high-speed) data transfer at rates in excess of 1.5 Megabits-per-second (Mbps) is expensive for dedicated bandwidth (e.g., leased lines) over the existing telecommunications infrastructure. Over modest ranges where an unobstructed line of sight exists, a laser communication link can provide an alternative means of obtaining dedicated bandwidth at high data rates.
For this and other reasons, wireless information transmission systems in general are increasingly desirable as alternatives to costly wired installations and high telecommunications rates which prevail even for short distance communications. Radio frequency communications systems have the disadvantage of requiring that carrier frequency and communications bandwidth be assigned to an application, since the much wider beamwidths and sidelobes can interfere with each other. Thus, there is an increasing need for communications systems, such as those using light frequencies, that transmit large quantities of information in a line-of-sight application without creating interference problems.
Microwave systems are also line of sight, but the majority of such systems on the market are not capable of the desired data rates in excess of 10 Mbps (e.g., 45, 100 and 155 Mbps). The higher-bandwidth microwave systems (i.e. &gt;10 Mbps) generally require Federal Communication Commission licensing, and are susceptible to terrain and building reflections, typically requiring tall towers which add significantly to the expense.
Free space laser communication within the atmosphere has been a realizable technology for many years, although commercially available laser communication systems are few in number. Manufacturers include Laser Communications Inc. (LCI) of Lancaster, Penn. (recently acquired by Jolt, Ltd. of Jerusalem, Israel), Proteon (formerly SilCom Technology) in Ontario, Canada, Canon USA in Englewood Cliffs, N.H. (Canobeam.TM.), and AstroTerra Corp. in San Diego, Calif. (TerraLink.TM.terminals). Each of these systems includes a basic set of subsystems: diode laser, beam-forming optics, receiver (telescope plus detector), electronics, and a mechanical housing.
The AstroTerra.TM. terminals, so far as the inventors are aware, are the longest range terminals commercially available at this time. The range of such competing systems is less than the range provided by the present invention, and because of their design systems of this type are inherently less reliable and more costly to manufacture than the system developed by the inventors which is described in more detail herein.
U.S Pat. No. 5,347,387 to Rice discloses a duplex optical transceiver wherein received light energy entering the system is initially reflected from a paraboloidal mirror onto a beam expander mirror. The light reflected from the beam expander then passes through a light baffle and a polarized beam rotating device. Next, the recollimated beam is multiplexed through a cube beamsplitter and re-imaged onto an avalanche photodiode detector. In the transmit path, the semiconductor laser emission is circularized and collimated to the desired beam divergence before being multiplexed through a cube beamsplitter and transmitted out the transceiver. Because Rice uses polarization to multiplex incoming and outgoing light beams, his system is limited as to the bandwidth of transmitted data.
U.S. Pat. No. 5,390,040 to Mayeux discloses an optical transceiver that employs a Cassegrain receiver for receiving an incoming beam reflected from a field of view onto the central portion of a primary mirror, and a transmitter for transmitting an outgoing beam of a different wavelength into the atmosphere. In Mayeux, the transmitted and received beams travel along two separate paths inside the transceiver.
U.S. Pat. No. 5,422,900 to Reele et al. discloses an optical compact disk writer with an integrated laser module wherein a laser driver circuit connected to the input of a laser is completely shielded from electromagnetic interference (EMI) from the rest of the system. The reference also discloses a lower housing formed of metal to act as the laser's main heat sink. U.S. Pat. No. 5,640,407 to Freyman et al. discloses a temperature regulating laser diode assembly wherein a laser diode is mounted directly on a compact thermoelectric cooler, which is then mounted on a heat sink.
Other references, such as U.S. Pat. Nos. 5,264,955 to Sakanaka et al., 5,424,860 to Mihara, 5,506,716 to Mihara et al., 5,535,034 to Tanaguchi, 5,142,400 to Solinsky, and 5,659,413 to Carlson show other communications technology applications.
None of these prior systems provides an optimal solution to the problem of establishing useful and reliable laser information transmission systems with significant range and bandwidth. Therefore, there is a need in the field for improved systems and methods which meet these needs.